1. Field of the Invention
The present invention relates generally to methods and systems for calibrating laser beam delivery systems, particularly ophthalmological surgery systems. More specifically, the present invention relates to methods and systems for calibrating a laser beam, such as a position or shape of the laser beam, from the laser beam delivery system using an image capture device.
Laser based systems are now commonly used in ophthalmological surgery on corneal tissues of the eye to correct vision defects. These systems use lasers to achieve a desired change in corneal shape, with the laser removing microscopic layers of stromal tissue from the cornea using a technique generally described as ablative photodecomposition to alter the refractive characteristics of the eye. Laser eye surgery techniques are useful in procedures such as photorefractive keratotomy (PRK), phototherapeutic keratectomy (PTK), laser in situ keratomileusis (LASIK), and the like.
Laser ablation procedures can reshape or sculpt the shape of the cornea for varying purposes, such as for correcting myopia, hyperopia, astigmatism, and other corneal surface profile defects. In known systems, the laser beam often comprises a series of discrete pulses of laser light energy, with the total shape and amount of tissue being removed being determined by the position, shape, size, and/or number of a pattern of laser energy pulses impinging on the cornea. A variety of algorithms may be used to calculate the pattern of laser pulses used to reshape the cornea so as to correct a refractive error of the eye.
Accurate control of the laser beam delivery system is crucial for patient safety and successful vision correction. Accordingly, laser beam delivery systems are calibrated to ensure control over the distribution of ablation energy across the cornea so as to minimize undesirable laser system performance, such as might result from flawed internal mechanical or optical components. In particular, calibration of the laser system helps ensure accurate removal of the intended shape and quantity of the corneal tissue so as to provide the desired shape and refractive power modification to the patient's cornea. Imprecise control of the laser beam may jeopardize the success of the surgery and could cause damage to the patient's eye. For example, derivation from a desired laser beam shape, size, or position, such as the laser beam exhibiting a non-symmetrical shape or an increased or decreased laser beam diameter, may result in tissue ablation at an undesired location on the patient's cornea which in turn leads to less than ideal corneal sculpting results. As such, it is beneficial to provide precise control over the shape and size profiles as well as positioning of the laser beam so as to accurately sculpt the patient's cornea through laser ablation.
Ablation of plastic test materials are often performed prior to laser surgery to calibrate the ablation shape and size of the laser beam delivery system. For example, an iris or other variable diameter aperture which may be used to tailor the shape, size, and position of the laser beam is typically calibrated by directing laser pulses at different iris settings onto a clear plastic material. Eye loops are then used by an operator for manual inspection of the ablated plastic. Such calibration techniques are limited by many factors, such as the precision provided by the eye loops, which is typically about ±0.1 mm, and/or the vision of the operator. For example, visual measurement of shape profiles is particularly difficult and is often subject to human error. Further, such calibration techniques may not accurately measure a hysteresis of the variable diameter iris. Moreover, increased utilization of wavefront technologies to provide customized ablations in laser eye surgery systems may be optimized by increasing the accuracy of the shape, size, and positioning of the ablating laser beam.
In light of the above, it would be desirable to provide improved methods and systems for calibrating laser beam positioning, shape profile, and/or size profile with increased precision and accuracy. It would be particularly desirable if such methods and systems provided for iris calibration as well as hysteresis measurement. It would be further desirable if such methods and systems enhanced calibration accuracy without significantly increasing the overall system cost and complexity. At least some of theses objectives will be met by the methods and systems of the present invention described hereinafter.
2. Description of the Background Art
Methods, systems, and apparatus for calibrating lasers are described in U.S. Pat. Nos. 6,195,164; 6,559,934; and 6,666,855, and assigned to the assignee of the present application. PCT Publication No. WO 01/10322 describes systems, devices, and methods for verifying the positioning or adjustment of a laser beam, and is also assigned to the assignee of the present application. Further laser calibration devices and methods are described in U.S. Pat. Nos. 3,364,493; 5,078,491; 5,261,822; 5,267,012; 5,772,656; 6,116,737; 6,129,722; 6,210,169; and 6,210,401.
The full disclosures of each of the above mentioned references are incorporated herein by reference.